Reflective optical elements for radiation in the ultraviolet wavelength range are used e.g. in microlithography projection exposure apparatuses for redirecting or bending of a laser beam with a given center wavelength (e.g. 193 nm). For such reflective optical elements, a high reflectance to the incident radiation over a wide range of incident angles is desirable. Moreover, the difference in amplitude and phase of the reflectivity of a polarization component with an electrical field strength vector parallel to a plane formed by the normal vector of the reflective surface and the direction of the incident beam (p-polarized radiation) and a polarization component perpendicular to that plane (s-polarized radiation) should be as small as possible. This is because, if the reflectivity of such a reflective optical element for s-polarized radiation is significantly different from the reflectivity for p-polarized radiation, the different intensity and phase of the two polarization components in the reflected beam tend to degrade the imaging performance of the projection exposure apparatus, if not being compensated for.
For maximizing reflectance, it is well-known to superimpose a dielectric multilayer system enhanced over the overall mirror surface. The multi-layer system comprises alternating layers of high refractive index layers and low refractive index layers, each having an optical thickness of λ/4 for a given incident angle. However, such a multilayer system does only yield high reflectance and a small difference between polarization components in a narrow range of incident angles.
In order to solve the above problems U.S. Pat. No. 6,310,905 discloses a reflective optical element with a dielectric multilayer system consisting of an arrangement of high refractive index layers alternating with low refractive index layers such that each high refractive index layer follows a low refractive index layer shown by the representation:L1/[H/L2]x  [1]wherein L1, L2: represent the low refractive index layersH: represents the high refractive index layersX: defines an integer between 1 and 10
The above formula [1] defines a dielectric multilayer system in which a succession of pairs of high refractive index layers H alternating with low refractive index layers L2 is superimposed over a first low refractive index layer L1. The letters used for the high and low refractive index layers H, L1, L2 are also representative for the optical thickness of these layers, such that different optical thickness may be expressed in terms of those letters, e.g. L1<L2. The letter X defines the repetition index, i.e. the number of times that the pair of layers HL2 is repeated in the multilayer system.
A mirror with a dielectric multilayer system similar to the one described above, albeit more complex, is disclosed in U.S. Pat. No. 5,850,309. In this system, several pairs of layers consisting of a high refractive index layer alternating with a low refractive index layer—being separated by so-called bonding layers—are repeated.